1. Field of the Invention
The invention generally relates to a communications system. More particularly, the invention relates to a communications device and a method of receiving radio frequency signals within a communications system.
2. Description of the Related Art
One example of a communications system is a wireless communications system which may be a cellular mobile communications system. The cellular mobile communications system is implemented in a geographical area and logically divided into individual service cells. A fixed transceiver station such as a base station defines at least one cell and is connected to a base station controller. Mobile stations, such as hand-held or car-based cellular phones, move freely within the geographical area covered by a cell.
The base station handles all telephone traffic to and from those cellular phones which are currently located in the cell. In addition, the cellular phones and the base stations exchange radio signals in accordance with a communications protocol defined for a given communications system. For instance, in a conventional Code Division Multiple Access (CDMA) systems, a traffic channel, a pilot channel, and a paging channel are defined for communications between the base stations and the cellular phones. The pilot channel carries no information, but provides the cellular phone, for example, with a reference for time, phase and signal strength. The cellular phone constantly evaluates the strengths of the pilot channels of the serving and neighboring base stations to determine potential base stations should the cellular phone move from one cell to another.
In some cellular phones, a receiver includes two frequency conversion stages. A first conversion stage down converts a received radio frequency (RF signal) to an intermediate frequency (IF) signal whose intermediate frequency (IF) is lower than the radio frequency. An amplifier amplifies the IF signal and a second conversion stage down converts the amplified IF signal to the baseband.
Conventional direct conversion (DC) module receivers, on the other hand, convert the RF signal directly down to the baseband. The DC module outputs an inphase (I) signal component and a quadrature (Q) signal component which are, in turn, processed by a baseband processor. Subsequent to the downconversion, amplifiers amplify the signal components I and Q in separate I- and Q-channels. The amplifiers should amplify the signal components I and Q with gains that are ideally the same for each one of the signal components I and Q. Unfortunately, gain imbalances between the I- and Q-channels may degrade the signal demodulation in the baseband processor.